1. Field of the Invention
The present invention relates to silicon-titanium mixed oxide powder prepared by flame hydrolysis, which is surface-enriched with silicon dioxide, and the preparation and use thereof.
2. Description of the Background
Titanium dioxide is widely used in sunscreen preparations. Its action substantially resides in the reflection, scattering and absorption of damaging UV radiation and is substantially dependent on the primary particle size of the metal oxides.
A disadvantage here is its photocatalytic activity, which triggers reactions which may give rise to changes in the constituents of a sunscreen preparation.
In an attempt to reduce the photocatalytic activity without diminishing the UV-screening properties, titanium dioxide is, for example, enclosed in a silicon dioxide shell.
The need for two reaction steps, with the correspondingly high capital cost, to prepare such enclosed particles is a disadvantage. The two steps incorporate the preparation of the titanium dioxide particles and the formation of the shell, generally in an aqueous (U.S. Pat. No. 6,773,814) or aqueous-alcoholic (U.S. Pat. No. 6,235,270) medium by the hydrolysis of a silicon dioxide precursor.
U.S. Pat. No. 5,451,390 describes a silicon-titanium mixed oxide prepared by flame hydrolysis and the use thereof as a UV-absorber in sunscreen preparations. Although this powder can be prepared in a single reaction step, the photoctalytic activity, in relation to titanium dioxide prepared by flame hydrolysis, is reduced to a lesser degree than is the case when titanium dioxide is enclosed in silicone dioxide.
Accordingly, there remains a need for a material which overcomes these disadvantages.